This invention relates to improvements in the making of hemispherical shells of "balata" for covering the core or center of a golf ball. The physical properties of balata articles have been known since at least 1848. The use of compounds having balata as the dominant constituent for the making of golf ball cover shells was specifically disclosed at least as early as 1933.
Balata is considered by chemists to be a "rubber." Analyses of rubber indicate the empirical formula (C.sub.5 H.sub.8)n; meaning a hydrocarbon made-up of a number of isoprene units. It has been determined that the isoprene units are linked head-to-tail; meaning a 1,4-chain polymer of isoprene.
The literature also refers to gutta-percha. Balata and gutta-percha are probably identical, and they have the same empirical formula as rubber. The balata hydrocarbon, like rubber, is a 1,4-chain polymer of isoprene, but has very different physical properties. Balata is a nonelastic, tough, thermoplatic solid; when heated and molded and cooled, it retains the shape of the mold cavity and does not retract to its original form. Balata can be vulcanized under conditions more drastic than those required for rubber, but without significant changes in physical properties.
Chemists using X-ray diffraction techniques have determinated a reason for the difference between the physical properties of rubber and balata or gutta-percha. It is now accepted in the art that the formulation of rubber is the cis form of a 1,4-chain polymer of isoprene, and the formulation of balata or gutta-percha is the trans form of a 1,4-chain polymer of isoprene.
The physical properties of balata, which provide a highly desirable compound for golf ball cover shells, have also compelled the art to use expensive and time-consuming techniques for both compound formulation and subsequent shell formation. At present, balata shells for golf ball covers are made using compression molding techniques. According to the invention, balata shells with optimum uniformity and lighter weight can be made faster, with reduced compound and direct labor costs, using injection molding techniques.